1. Field of the Invention
The present invention pertains to organic compounds of the tetrathiafulvalene type which can be used to make Langmuir-Blodgett conducting films.
The Langmuir-Blodgett method makes it possible to obtain very thin (10 to 40 angstroms) organized monolayers on the surface of a liquid, to transfer these monolayers and to superimpose them on a substrate. The possibilities for the application of these films are derived from the following characteristics: simplicity of the preparation method, very high precision of thicknesses obtained (a few angstroms), orientation of the molecules and adjustable physical properties.
2. Description of the Prior Art
Langmuir-Blodgett films are obtained from amphiphilic molecules. These molecules are elongated and have a hydrophilic group at one end and a hydrophobic group at their other end. The simplest amphiphilic molecules are the fatty acids CH.sub.3 --(CH.sub.2).sub.n --COOH which consist of a hydrophilic polar head --COOH and a hydrophobic aliphatic chain CH.sub.3 --(CH.sub.2).sub.n --. Behenic acid CH.sub.3 --(CH.sub.2).sub.20 --COOH is one of the most frequently used molecules.
The very small thickness of these films makes them especially valuable in micro-electronics. It has therefore, been sought to make them conduct electricity. EP No. 193 362 describes conducting films of this type made with tetracyanoquinodimethane (TCNQ) suitably substituted so that it can be deposited in the form of a Langmuir-Blodgett film.
Another type of conductivity would be of greater value in electronics. This is anisotropic conductivity, especially when the anisotropism is such that the conduction takes place in the direction of the plane of the film while the insulation is provided in a direction perpendicular to this plane.
Organic materials having a conduction anisotropism are already known. Tetrathiafulvalenes (TTF) are electron donor organic compounds which can give rise to the formation of complex compounds through the transfer of charges with electron acceptors. Compounds of this type, known as organic metals, have the remarkable property of conducting electrical current in a preferred direction. In view of the value of these compounds, especially in electronics, a great many works have been devoted to studying their synthesis and their properties (M. Narita, C. U. Pittman Jr., Synthesis 1976, p. 489; J. B. Torrance, Account Chemical Research 1979, No. 12, p. 79, F. Wudl, Account Chemical Research 1984, No. 17, p. 227). However, these molecules are not amphiphilic and, therefore, cannot lead to the formation of Langmuir-Blodgett films. To date, no method is known for the preparation of amphiphilic TTF molecules.
In order to cope with this problem, the invention proposes new molecules of tetrathiafulvalene which are disubstituted so as to be made amphiphilic. It is thus possible to obtain Langmuir-Blodgett thin films which become conductive when they are doped by electron acceptors (iodine, bromine, arsenic pentafluoride, etc.). Furthermore, the layered structure of these films results in a conductivity anisotropism: they are conductive in the directions of the plane of the film and insulating in the direction perpendicular to this plane.